Fine particle magnets



United States Patent e 1 2,967,794 lgfi Patented Jan. 10 1961 FINE PARTICLE MAGNETS Charles D. Coxe, Fairfield, Conn., assignor to Handy & Harman, New York, N.Y., a corporation of New York No Drawing. Filed Sept. 12, 1956, Scr. No. 609,404

12 Claims. c1. 14s 101 This invention relates to ferromagnetic materials and to methods of manufacture thereof, and more particularly to materials comprising ferromagnetic metal or alloy particles in a finely subdivided state which exhibit a high degree of magnetic shape anisotropy. The invention provides materials in which ferromagnetic particles having high length-to-diametcr ratio and an extremely small diameter are produced, with a high degree of alignment of the long axis of the particles and with good magnetic insulation between the particles, especially in directions transverse to their length. The invention comprises materials in which these particles are supported and dispersed through a non-magnetic metallic matrix, or through a substituted matrix preferably of non-metallic material. The invention also comprises the wool-like aggregate of oriented elongated ferromagnetic particles themselves substantially without matrix.

Materials according to the invention may find application as either hard or soft magnetic materials, a hard magnetic material being one suitable for use as a permanent magnet and a soft magnetic material being one suitable for use as a magnetic core in an alternating current device. The materials of the invention are suitable for permanent magnet uses when in accordance with the process of the invention a large proportion of the ferro-magnetic particles therein, which are of generally filamentary shape, are reduced in diameter to diameters so small that the particles constitute effectively single domain particles. The materials of the invention are suitable for use as soft magnetic materials at somewhat larger limits of particle diameter.

According to the general method of the invention, finely divided metallic powder of ferromagnetic nature, for example iron powder or powder of iron-nickel, ironcobalt or iron-nickel-cobalt alloys, is thoroughly mixed with a non-magnetic metallic powder such as silver or copper having minimum solubility with the ferromagnetic material employed. The non-magnetic powder is preferably of equal or greater fineness. This non-magnetic metal acts as a matrix for the ferromagnetic particles during the remainder of the processing, and may be left in place in the final product.

The proportions in the mixture are determined by the volume percentage of ferromagnetic material desired in the end product, which may according to the invention vary between substantially and 75%. In measuring the volumes of powder for the initial mixture, account should be taken of differences if any between the packhundred or more and the length of the billet being increased by the square of this factor.

Advantageously the first steps in this cold working process comprise one or more swaging operations followed by one or more drawing operations. When the diameter of the wire into which the billet has been drawn becomes so small as to increase disproportionately the cost of further drawing operations, the wire may be cut into lengths which may then be bundled and enclosed in a sheath of the silver or other matrix material for renewed working by swaging and drawing. Alternatively the bundles of wire lengths may be hot pressed or sintered to produce a new billet for renewed working.

By means of such a two-stage process the diameter of the ferromagnetic particles in the billet may be reduced by a factor of the order of 2,500, the initially equiaxed or approximately spherical particles of the starting pow ders being in consequence greatly elongated. The resulting material contains particles of ferromagnetic material with an extremely high length-to-diameter ratio, of the order of magnitude of 1,000,000 or more, provided the annealing steps are performed at temperatures which avoid objectionable grain growth and coalescence of the ferromagnetic particles. The elongated particles thus produced have a very high degree of alignment and are substantially uniformly packed or spaced through the nonferromagnetic matrix.

An example of the procedure according to my inven tion is as follows: Commercial electrolytic iron power of through 325 US. standard screen mesh size (nominal particle diameter 25 microns) is thoroughly mixed with an equal volume of silver powder of the same or greater fincness. If exactly fifty volume percent of ferromagnetic material is desired in the product of the cold working operations, the volume of the silver powder employed may be corrected to compensate for the unequal packing factors applicable to the two powders. This mixture is pressed at 20,000 pounds per square inch into a bar which is sintered for one hour at 1,700 F. in hydrogen to produce a billet approximately 1 inch square and 10 inches long. This is worked into wire of 0.020 inch diameter as follows:

Swage 1 inch square to /s inch round.

Anneal at l,400 F.

Swage A; inch round to /1 inch round.

Anneal at 1,400 F.

Swage inch round to V2 inch round.

Anneal at l,100 F.

Draw /2 inch round to 0.200 inch round.

Anneal at 1,100 F.

Draw 0.200 inch round to 0.062 inch round.

Anneal at l,000 F.

1 Draw 0.062 inch round to 0.020 inch round.

Anneal at l,000 F.

The annealing times are held to the minimum consistent with satisfactory workability of the material Annealing times of the order of magnitude of one hour have proven satisfactory. The annealing is carried out in hydrogen or in any other atmosphere which is reducing to iron at the applicable temperature. The wire of 0.020 inch diameter thus produced is cut into 10 inch lengths and collected into a bundle of the order of one square inch cross section. This bundle is sheathed in a silver tube and subjected to the same working schedule as that set forth above except that the annealing temperature is held below l,000 F. in all cases.

The resulting double drawn wire contains aligned iron particles having a diameter of the order of 0.01 micron and a length several million times as great, with a substantial degree of magnetic insulation between the particles. While the particles in the iron powder starting material are far above single domain size, the diameter 3. of the filamentary iron particles in the double drawn wire is so small that filaments thereof behave substantially like single domain particles, these diameters being of the order of magnitude of 100 or 200 angstrom units. Because the length-to-diameter ratio of these filaments is high, being higher than ten and in fact of the order of the particles exhibit a high degree of magnetic shape anisotropy. They consequently exhibit the properties of hard ferro-magnetic materials including high rcmanence, high coercive force and high B.H. max. energy product values. The energy product value is particularly high in materials containing approximately fifty volume percent of iron. Consequently the double drawn wire is useful as a permanent magnet material, permanent magnets of a wide variety of shapes being readily produced therefrom either with the wire in its drawn shape or after flattening into a ribbon for example.

In working with copper as a matrix material the process of the invention is substantially the same except that very slow cooling should be employed after the sintcr ing. Annealing temperatures should also be held to the lowest possible levels in view of the greater solubility of iron and copper in each other.

Soft magnetic materials of similar type may be produced according to the invention by halting the drawing process when the particle diameter reaches a dimension of the order of magnitude of 1 micron or larger, at which level the particles do not exhibit single domain behavior.

The invention also includes materials in which the copper or silver or like metallic matrix, selected for its mutual insolubility with iron and for its resistance to grain growth of the iron, is removed, for example by use of a suitable solvent. This is particularly desirable in the case of soft magnetic materials which are to be subjected to an AC. magnetic field.

Thus a silver matrix, as described in the example of the invention above set forth, may be removed by selective dissolution in a solution of sodium cyanide, 10% by weight in water. Mercury may be used to remove the silver instead. The dissolution is conducted without excessive stirring in order to leave the elongated ferromagnetic fibers with their parallel alignment preserved.

The fibers are then removed in an aggregate resembling textile sliver. After washing this may be impregnated, for example by a spraying operation, with a plastic and electrically non-conductive binder. The resulting product may then be compressed to any desired degree. Or the wool-like aggregate may be immersed in a plastic material such as a monomeric resin which can then be polymerized. If electrical conductivity in the mass of the product is not objectionable for its contemplated use, the copper or silver matrix employed in the production of the elongated ferromagnetic fibers may be replaced With a non-alloying metal matrix of low melting point such as lead. This may be desirable for reasons of economy as for example in the case of a silver matrix. Alternatively the product of the drawing operations in the form of a wire may be compacted and sintered or cemented, care being taken to avoid temperatures which will promote grain growth.

The materials of the invention, whether for use as hard or soft magnetic materials, are magnetized in directions parallel or substantially parallel to the direction of elongation of the cold Worked billets from which they are taken, i.e. parallel to the long dimensions of the ferromagnetic particles therein. In the case of permanent magnets this may be done as a step in manufacture thereof with a DC. field aligned parallel with the long dimension of these particles. Soft magnetic materials are employed so that their disposition, for example within a coil, aligns the successively opposite directions of the alternating field produced by the coil parallel to this long dimension.

While the invention has been described herein in terms of the presently preferred embodiments and practice thereof, it is not limited to the details of procedure hereinabove set forth. Thus the starting powders may be of particles sizes other than the 25 micron figure mentioned in connection with the example of the invention given. Particle sizes of the order of 50 microns for the ferromagnetic material are satisfactory as are also of course smaller particle sizes for example of the order of 5 microns. Neither is the invention limited to the particular values of ultimate particle diameter which have been mentioned. Useful magnetic materials according to the invention may also result from working schedules in which the factor by which the diameter of the billet is reduced, which represents substantially the factor by which the diameter of the ferromagnetic particles is reduced, is such as to reduce the diameter of these particles to values between 1 micron and .01 micron, for example to 0.1 micron, or to diameters below .01 micron. The optimum diameter of the ferromagnetic particles in the materials of the invention will depend to some extent, especially in the case of materials intended for use as permanent magnets, upon the composition of the ferromagnetic phase.

I claim:

1. A wrought magnetic material comprising metallic filamentary ferromagnetic particles having a high lengthto-diameter ratio, said particles being oriented with their long dimensions substantially parallel and being dispersed through a solid non-magnetic matrix.

2. A wrought magnetic material comprising metallic filamentary feromagnetic particles having a length-to-diameter ratio in excess of and a diameter less than about 1 micron, said particles being oriented with their long dimensions substantially parallel and being dispersed through a metallic matrix comprising a metal drawn from the group consisting of copper and silver.

3. A wrought magnetic material comprising metallic filamentary ferromagnetic particles having a length-todiameter ratio in excess of 100 and a diameter less than about 1 micron, said particles being oriented with their long dimensions substantially parallel and being dispersed through a metallic matrix consisting of a metal drawn from the group consisting of silver, copper, and alloys thereof.

4. A sintered and wrought magnetic material comprising parallel oriented fibers of ferromagnetic material of single domain diameter having a length-to-diameter ratio in excess of 100 in a metallic matrix comprising a metal drawn from the group consisting of silver and copper.

5. A process for manufacturing ferromagnetic material including aligned ferromagnetic particles having a length-to-diameter ratio in excess of 100 and a diameter of the order of 1 micron comprising the steps of intimately mixing powdered ferromagnetic particles with a non-ferromagnetic powder to produce a mixture containing substantially from 5% to 75% of ferromagnetic pow der by volume, compacting and sintering said mixture into a billet, and cold working said billet by repeated drawing and annealing operations to reduce the diameter of said ferromagnetic particles to a dimension of the order of magnitude of 1 micron.

6. The process of manufacturing ferromagnetic material including aligned elongated ferromagnetic particles which comprises the steps of intimately mixing a finely divided ferromagnetic metallic powder with a metallic powder of equal or greater fineness drawn from the group consisting of silver, copper and alloys thereof to product an initial mixture containing substantially from 5% to 75% by volume of ferromagnetic material, compacting said mixture into a billet, sintering said billet in a reduc .ing atmosphere, and cold working said billet by repeated drawing and annealing operations to reduce the diameter thereof by a factor at least as great as the ratio of the mean diameter of the ferromagnetic particles in said initial mixture to 1 micron.

7. The process of manufacturing ferromagnetic material including aligned elongated ferromagnetic particles which comprises the steps of intimately mixing a finely divided ferromagnetic metallic powder with a metallic powder of equal or greater fineness drawn from the group consisting of silver, copper and alloys thereof to produce an initial mixture containing substantially from 5% to 75% by volume of ferromagnetic material, compacting said mixture into a billet, sintering said billet in a reducing atmosphere, cold working said billet by repeated drawing and annealing operations to reduce the diameter thereof by a factor at least as great as the ratio of the mean diameter of the ferromagnetic particles in said initial mixture to 1 micron, and selectively dissolving out the non-ferromagnetic material from the product of said cold working step while preserving the parallel orientation of the elongated ferromagnetic particles produced by said cold working.

8. The process of manufacturing ferromagnetic material including aligned elongated ferromagnetic particles which comprises the steps of intimately mixing a finely divided ferromagnetic metallic powder with a metallic powder of equal or greater fineness drawn from the group consisting of silver, copper and alloys thereof to produce an initial mixture containing substantially from 5% to 75% by volume of ferromagnetic material, compacting said mixture into a billet, sintering said billet in a reducing atmosphere, cold working said billet by repeated drawing and annealing operations to reduce the diameter thereof by a factor at least as great as the ratio of the mean diameter of the ferromagnetic particles in said initial mixture to 1 micron, selectively dissolving out the non-ferromagnetic material from the product of said cold working step while preserving the parallel orientation of the elongated ferromagnetic particles produced by said cold working, and impregnating the ferromagnetic residue of said dissolving step with a non-magnetic material.

9. The process of manufacturing ferromagnetic material including aligned elongated ferromagnetic particles which comprises the steps of intimately mixing a finely divided ferromagnetic metallic powder with a metallic powder of equal or greater fineness drawn from the group consisting of silver, copper and alloys thereof to.

produce an initial mixture containing substantially from 5% to 75 by volume of ferromagnetic material, compacting said mixture into a billet, sintering said billet in a reducing atmosphere, cold working said billet into wire, and drawing said wire to reduce the diameter of said billet by a factor at least as great as the ratio of the mean diameter of the ferromagnetic particles in said initial mixture to 1 micron.

10. The process of manufacturing ferromagnetic material including aligned elongated ferromagnetic particles which comprises the steps of intimately mixing a finely divided ferromagnetic metallic powder with a metallic powder of equal or greater fineness drawn from the group consisting of silver, copper and alloys thereof to produce an initial mixture containing substantially from 5% to 75% by volume of ferromagnetic material, compacting said mixture into a billet, sintering said billet in a reducing atmosphere, cold working said billet into wire, drawing said wire, cutting and bundling said drawn wire into a new billet, and similarly working said new billet until the total reduction factor in the diameter applicable to said original billet is at least as great as the ratio of the mean diameter of the ferromagnetic particles in said initial mixture to 1 micron.

11. A process for manufacturing permanent magnet material including aligned ferromagnetic particles having a length-to-diameter ratio in excess of 100 and a diameter of the order of .01 micron comprising the steps of intimately mixing substantially equal volumes of powdered ferromagnetic particles having equiaxed dimensions of microns or less and non-ferromagnetic powder of equal or greater fineness, compacting and sintering said mixture into a billet, cold working said billet by repeated drawing and annealing operations to reduce the diameter of said ferromagnetic particles to a dimension of the order of magnitude of .01 micron, and magnetizing said material in a direction substantially parallel to the length of said particles.

12. A process for manufacturing ferromagnetic material including aligned ferromagnetic particles which comprises the steps of intimately mixing powdered ferrw magnetic particles having substantially equiaxed dimensions of 50 microns or less and non-ferromagnetic powder of equal or greater fineness to produce a mixture containing substantially from 5 percent to percent of ferromagnetic powder by volume, compacting and sintering said mixture into a billet, cold working said billet by repeated drawing and annealing operations to increase the length-to-diameter ratio of said ferromagnetic particles to at least ten, and selectively dissolving out the nonferromagnetic material from the product of said cold working step while preserving the parallel orientation of the elongated ferromagnetic particles produced by said cold working.

References Cited in the file of this patent UNITED STATES PATENTS 1,991,143 Ehlers Feb. 12, 1935 2,085,118 Noll June 29, 1937 2,499,977 Scott Mar. 7, 1950 2,619,438 Varian et a1 Nov. 25, 1952 2,651,105 Neel Sept. 8, 1953 2,677,782 Gehrke et al. May 4, 1954 2,685,568 Wilson Aug. 3, 1954 2,715,109 Albers et al. Aug. 9, 1955 FOREIGN PATENTS 431,511 Great Britain July 5, 1935 OTHER REFERENCES Permanent Magnets From Ultrafine Iron Powder, Bernard Kopelman, Electrical Engineering, May 1952, pp. 447-451. 

1. A WROUGHT MAGNETIC MATERIAL COMPRISING METALLIC FILAMENTARY FERROMAGNETIC PARTICLES HAVING A HIGH LENGTHTO-DIAMETER RATIO, SAID PARTICLES BEING ORIENTED WITH THEIR LONG DIMENSIONS SUBSTANTIALLY PARALLEL AND BEING DISPERSED THROUGH A SOLID NON-MAGNETIC MATRIX.
 5. PROCESS FOR MANUFACTURING FERROMAGNETIC MATERIAL INCLUDING ALIGNED FERROMAGNETIC PARTICLES HAVING A LENGTH-TO-DIAMETER RATIO IN EXCESS OF 100 AND A DIAMETER OF THE ORDER OF 1 MICRON COMPRISING THE STEPS OF INTIMATELY MIXING POWDERED FERROMAGNETIC PARTICLES WITH A NON-FERROMAGNETIC POWDER TO PRODUCE A MIXTURE CONTAINING SUBSTANTIALLY FROM 5% TO 75% OF FERROMAGNETIC POWDER BY VOLUME, COMPACTING AND SINTERING SAID MIXTURE INTO A BILLET, AND COLD WORKING SAID BILLET BY REPEATED DRAWING AND ANNEALING OPERATIONS TO REDUCE THE DIAMETER OF SAID FERROMAGNETIC PARTICLES TO A DIMENSION OF THE ORDER OF MAGNITUDE OF 1 MICRON. 